How to Calculate How Much Yarn You Need
Ever bought yarn by eye and ended up halfway through a scarf with nothing left? Or realised mid-project that you're one ball short - and that colourway is long discontinued? Calculating the meterage upfront is a small step that saves a lot of frustration.
Why do patterns use meters, not grams?
Grams depend on thickness. 100 g of lace-weight mohair can give you several hundred metres, while 100 g of chunky roving gives you barely fifty. Meterage is the universal language - it means the same thing regardless of brand, fibre, or weight. That's why every well-written pattern specifies metres (or yards), not balls.
How to read a yarn label
Every label carries two numbers you need for the calculation:
- ball weight – e.g. 100 g
- ball length – e.g. 200 m
The ratio between them is all you need. In this example: 2 metres per gram.
Formula: how many metres do you have?
If you have several balls or a single unwound skein, calculate like this:
total metres = (your grams ÷ ball weight) × ball length
Example: you have 250 g of yarn; each ball weighs 100 g and contains 200 m.
(250 ÷ 100) × 200 = 500 m
You have 500 metres - enough for a typical adult scarf.
Formula: how many balls to buy?
Flip the calculation:
balls needed = metres required by pattern ÷ metres per ball
Always round up - one extra ball is far better than running out halfway through.
Example: the pattern calls for 380 m, each ball has 200 m.
380 ÷ 200 = 1.9 → buy 2 balls
Calculate in seconds - no pen and paper needed
If you'd rather skip the manual maths, I built a dedicated tool for exactly this. Enter the label data, add your grams or the pattern's meterage - the calculator does the rest.
The tool works in two modes:
- "How many metres do I have?" - converts your grams to metres
- "How many balls to buy?" - works out the number of balls and total weight needed
One important caveat: ±5–10% tolerance
The meterage on the label is a nominal value. In practice, different dye lots of the same yarn - or even individual balls within one lot - can vary by 5–10%. To stay safe:
- Always buy one ball more than your calculation suggests.
- For larger projects (blankets, sweaters) order everything from the same dye lot - the lot number is printed on the label.
Summary
To calculate how much yarn you need, you only have to know three numbers: the ball weight, the ball length, and how many grams you have (or how many metres the pattern requires). The ratio from the label converts one to the other in seconds - either with the formula above, or with the calculator.